The present invention relates to a method and apparatus for control of a polishing process based on a signal peak analysis, in particular to a method and apparatus for monitoring a polishing process, such as chemical mechanical polishing, in the manufacture of semiconductor wafers.
Polishing processes play significant role in modern technologies, in particular in semiconductor fabrication. For example, at certain stages in the fabrication of devices on a substrate, it may become necessary to polish the surface of the substrate before further processing may be performed. In a polishing process, known as mechanical polishing, a polishing pad with abrasive particles repetitively passes over the surface of the substrate. Polishing may also be performed with chemically active abrasive slurry. A polishing system that uses the chemical slurry is commonly known as a chemical mechanical polishing (CMP) system. In contrast with mechanical polishing, the slurry in a CMP system provides an increased removal rate of a substrate material. Additionally, by selecting particular chemicals, chemical slurry can be used to selectively polish certain films on a semiconductor substrate.
Wafer polishing typically requires that a substrate be mounted in a wafer head or carrier, with the surface of the substrate to be polished exposed. The substrate supported by the head is then placed against a moving polishing pad mounted on a platen. The head holding the substrate may also move, e.g., rotate, to provide additional motion between the substrate and the polishing pad surface. In a CMP process, a polishing slurry (typically including abrasive particles and at least one chemically reactive agent, which is selected to enhance the polishing of the topmost film layer of the substrate) is supplied to the pad to provide an abrasive chemical solution at the interface between the pad and the substrate.
Both the CMP and traditional mechanical abrasion polishing are difficult to control. The CMP process is frequently carried out without comprehensive information about current polishing conditions, and its control is performed just by using empirical polishing rates and timing. Since such polishing control methods are inaccurate, the polishing process results in serious yield drops and waste of expensive wafers. Therefore, accurate control of polishing based on reliable real-time information is an important issue for automation of such processes.
One of known methods used to control polishing is based on measurements of sound waves or audible acoustic noise generated in the interface between the wafer and polishing material (e.g., three U.S. Pat. No. 5,222,329 issued on Jun. 29, 1993, U.S. Pat. No 5,245,794 issued on Sep. 21, 1993, and U.S. Pat. No. 5,439,551 issued on Aug. 8, 1995).
This control method is based on the fact that changes in the polishing process are accompanied by noticeable changes in the amplitude and spectrum of an acoustic signal. The acoustic signals are detected with the use of an acoustic signal receiver, such as a microphone, located in the vicinity, but outside of the polishing zone of a wafer-pad contact. The signal is then recorded in a manner known in the art and used for a process analysis. This method, however, is limited to measuring low frequency signals, which reduces its sensitivity, and to monitoring their average characteristics, which does not allow for detection of local polishing defects such as delamination and scratches.
U.S. Pat. No. 5,876,265 issued on Mar. 2, 1999 to T. Kojima describes a polishing apparatus with end point detection based on analysis of acoustic signals involving peak detection. The polisher includes a vibration detecting device attached to a wafer carrier, an amplifier, a gain indicator with gain adjuster to adjust a gain of such amplifier, an end-point analyzer based on the change in said adjusted signal, and a controller to stop the polishing motion upon the end-point signal. The suitability of the amplifier gain and need for its adjustment is determined based on whether it""s either amplitude or maximum value resides in a preset reference range. The gain indicator may include a peak detector for detecting peaks of signal waveform and making gain adjustments based on a peak signal output when a magnitude is not suitable. These method and apparatus, however, measure only low-frequency signals, which makes them less sensitive to changes in the polishing process. Also, they do not allow making a distinction between regular acoustic signals and those generated due to abnormal polishing effects, such as scratches and delamination.
Another example of polishing control based on detection of acoustic signals is disclosed in U.S. patent application Ser. No. 09/859,062 filed by the same applicants on May 17, 2001. The present patent application is a continuation-in-part of the aforementioned patent application.
The above patent application describes an apparatus for controlling a CMP process, comprising a rotating or orbiting platen with a pad, a rotating head that supports an object to be treated, e.g., a semiconductor wafer, and performs radial movements with respect to the platen, and a polishing process control system comprising a plurality of groups of various sensing devices for detecting changes that occur during polishing. One group of the sensing devices is a group of high-frequency acoustic emission sensors built on various levels into components of the rotating head. Another group of sensing devices is represented by force/torque sensors connected with various elements of the rotating head and the platen, respectively, and intended for direct measurement of compression force and friction response (force or torque) between the head and the platen and a coefficient of friction between the wafer and the polishing pad. All groups of sensors work simultaneously and their measurement data is processed and analyzed by a control unit for obtaining accurate and reliable results. The results are then used for adjusting the process.
One disadvantage of all known polishing control methods and apparatus is that they do not provide quantitative evaluation of changes in a polishing process in a manner required for optimization of the process and for preventing occurrence of defects that might be caused by the polishing process itself. Another disadvantage consists in that known polish control methods just establish the fact of occurrence of the changes and do not distinctly distinguish between regular peaks of signals and extraordinary peaks. Such extraordinary peaks may be caused by undesired events such as delamination of sublayers within a semiconductor wafer resulting, e.g., from an excessive normal force, with which the polishing pad is pressed to the surface of the wafer being treated.
It is an object of the present invention to provide a method and apparatus for polishing control with quantitative evaluation of changes in the polishing process. Another object is to provide a method and apparatus for automatic analysis of acoustic or other signals and for controlling the process in a manner that allows to optimize polishing conditions and thus to prevent occurrence of defects during polishing. Another object is to provide a method and apparatus capable of separating extraordinary signal peaks from regular signal peaks.